Thymidylate synthase (TS) is the molecular target of fluoropyrimidine (FP) chemotherapy, and novel anticancer drugs effective against TS-overexpressing tumors are required to treat patients with FP-refractory solid tumors. We have evaluated the inhibition of cell proliferation in vitro and antitumor activity in vivo of FdUMP[10], an oligodeoxynucleotide 10mer in which 5-fluorouracil (5-FU) is the only nucleobase. FdUMP[10] is a pro-drug of FdUMP, the TS inhibitory metabolite of FPs. FdUMP[10] was 338-fold more potent than 5-FU at inhibiting cell proliferation in the NCI 60 cell line screen. The antitumor activity of FdUMP[10] was compared to 5-FU using H-T29 xenografts in female CD-1 athymic (nu+/nu+) mice. Treatment with FdUMP[10] as a single agent (40 mg/kg/daily x 5, i.v.) delayed tumor growth and resulted in a smaller mean tumor size (T/C value = 51%, p<0.001 compared with the control group). Treatment with 5-FU (25 mg/kg/daily x 5, i.p.) had similar results as single agent FdUMP[10] (T/C value = 65%, p=0.238 compared with the FdUMP[10] treated group. Simultaneous treatment of tumor-bearing mice with both drugs (FdUMP[10] plus 5-FU) further delayed tumor growth (T/C value = 36%; p=0.003 relative to 5-FU). The results from the combined treatment group were not, however, statistically significant relative to the group receiving single agent FdUMP[10] treatment (p=0.059). Histological examination revealed systemic damage was limited to the colonic epithelium in all treatment groups and was least extensive with single agent FdUMP[10] compared to the other treatment groups. The data support the concept that FdUMP[10] is a useful prototype of a novel type of FP that is likely to be more efficacious than FPs in clinical use.

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